Bicapped Tetrahedral, Trigonal Prismatic, and Octahedral Alternatives in Main and Transition Group Six-Coordination
2484 Bicapped Tetrahedral, Trigonal Prismatic, and Octahedral Alternatives in Main and Transition Group Six-Coordination Roald Hoffmann,*laJames M. Howell,lb and Angelo R. RossilC Contribution from the Departments of Chemistry, Cornell University, Ithaca, New York 14853, Brooklyn College, City University of New York. Brooklyn, New York, 1121 0, and The University of Connecticut, Storrs, Connecticut 06268. Received June 30, 1975 Abstract: We examine theoretically the bicapped tetrahedron and trigonal prism as alternatives to the common octahedron for six-coordinate main and transition group compounds. The pronounced preference of six-coordinate sulfur for an octahe- dral geometry is traced by a molecular orbital analysis to a pair of nonbonding levels, whose higher energy in the nonoctahe- dral geometries is due to the molecular orbital equivalent of a ligand-ligand repulsion. For transition element complexes we draw a correlation diagram for the trigonal twist interrelating octahedral and trigonal prismatic extremes. A possible prefer- ence for the trigonal prism in systems with few d electrons is discussed, as well as a strategy for lowering the energy of the trigonal prism by symmetry-conditioned 7r bonding. The bicapped tetrahedron geometry for transition metal complexes can be stabilized by a substitution pattern ML4D2 where D is a good u donor and M is a d6 metal center. In main and transition group six-coordinate compounds logical, or group theoretical analysis of rearrangement the octahedral geometry predominates. There is, however, a modes or basic permutational sets. However, the solution of growing class of trigonal prismatic complexes and struc- the mathematical problem leaves the question of the physi- tures intermediate between the octahedron and trigonal cal mechanism of a rearrangement still unsolved.
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